Tribology and Lubrication Technology December 2012 : Page 21

STudEnT PoSTER abSTRacT Real-Time TEm imaging of compression and Shear of Single fullerene-Like moS 2 and WS 2 nanoparticles Imène Lahouij -Ecole Centrale de Lyon, Laboratory of Tribology and System Dynamics, Ecully, France Fabrice Dassenoy -Ecole Centrale de Lyon, Laboratory of Tribology and System Dynamics, Ecully, France Beatrice Vacher -Ecole Centrale de Lyon, Laboratory of Tribology and System Dynamics, Ecully, France Jean-Michel Martin -Ecole Centrale de Lyon, Laboratory of Tribology and System Dynamics, Ecully, France Note: For a closer look at Imène’s poster abstract, be sure to check out her short video presentation in the December digital version of TLT (available at www.stle.org) inTRoDucTion Inorganic fullerene-like (IF) MoS 2 and WS 2 nanoparticles have received increasing attention in tribology applications as new generation of friction modifier and antiwear additives in lubricating oil. 1,2 However due to their very small size, the effect of the intrinsic parameters (size, morphology, etc.) of the nanoparticles on their lubricating properties are exceed-ingly difficult to identify. In this work a new in situ transmission electron micro-scope TEM technique including Nano indentation is used to manipulate individual IF-MoS 2 and IF-WS 2 nanoparticles. Thanks to this system, the deformation under pressure and shear of single nanoparticles is observed in real-time in the TEM and related in the force time curve. The load-depen-dent behavior of structural changes of nanoparticles has been investigated and critical values of pressure for exfolia-tion and rolling of some individual nanoparticles have been measured. mATERiALS AnD mEThoDS Imène Lahouij is a doctoral student in the Tribol-ogy and System Dynamics Laboratory at the Ecole Centrale de Lyon in France, working under the guidance of professor Fabrice Dassenoy. She is involved in the Addnano project for development of new generation of fluid lubricants incorporat-ing nanomaterials. Her research interests include nanotribology, surface engineering and electron microscopy. You can reach her at imen.Lahouij@ ec-lyon.fr. WWW .S TLE. OR G The compression experiments were carried out using a Nanofactory Instruments HN200 single tilt Nanoindenter Micro-Force probing holder3 mounted on the specimen holder of a JEOL 2010 FEG microscope operating at 200 kV accelerating voltage. The nanoindentation sensor used for the experiments was equipped with a truncated shaped diamond tip. The force range of the sensor was between 0 and 3 mN and its stiffness constant was 3500 N/m with a force resolution of 100 nN. The TEM-Nanoindentor permits to image the Nano compression process in real time, to record movies, as well TRIB OL OG Y & L UBRIC A TION TE CHNOL OG Y DE CEMBER 2 012 • 21

Student Poster Abstract

Real-Time TEM imaging of compression and Shear of Single fullerene-Like moS2 and WS2 nanoparticles

INTRODUCTION
Inorganic fullerene-like (IF) MoS2 and WS2 nanoparticles have received increasing attention in tribology applications as new generation of friction modifier and antiwear additives in lubricating oil.1,2 However due to their very small size, the effect of the intrinsic parameters (size, morphology, etc.) of the nanoparticles on their lubricating properties are exceedingly difficult to identify.

In this work a new in situ transmission electron microscope TEM technique including Nano indentation is used to manipulate individual IF-MoS2 and IF-WS2 nanoparticles. Thanks to this system, the deformation under pressure and shear of single nanoparticles is observed in real-time in the TEM and related in the force time curve. The load-dependent behavior of structural changes of nanoparticles has been investigated and critical values of pressure for exfoliation and rolling of some individual nanoparticles have been measured.

MATERIALS AND METHODS
The compression experiments were carried out using a Nanofactory Instruments HN200 single tilt Nanoindenter Micro-Force probing holder3 mounted on the specimen holder of a JEOL 2010 FEG microscope operating at 200 kV accelerating voltage.

The nanoindentation sensor used for the experiments was equipped with a truncated shaped diamond tip. The force range of the sensor was between 0 and 3 mN and its stiffness constant was 3500 N/m with a force resolution of 100 nN. The TEM-Nanoindentor permits to image the Nano compression process in real time, to record movies, as well as to acquire the force-displacement and force-time data. The method used to estimate the pressure contact corresponding to the applied load on the particle during compression and sliding tests is described in (I. Lahouji, et.al.).

For compression and sliding experiments, substrates on which the nanoparticles have to be previously deposited must be nano-machined to be thin enough for electron transparency in one dimension, long enough in second dimension and moderately short in the third one, in order to avoid crashing onto the sensor. The substrate was 7 µm wide and 100 µm long. Additional 10 µm long steps were machined to have platelets for the nanoparticle deposition (see Figure 1). The details of the substrate preparation have already been reported in (I. Lahouji, et.al.).

The experiments were conducted with IF-MoS2 and IFWS 2 nanoparticles supplied by Professor Tenne’s group at Weizmann Institute in Israel. The particles were synthesized by gas-phase reaction between oxides (MoO3 or WO3) and H2S.5,6 The IF-MoS2 particles have a perfectly structure without any defect and a high crystalline order presenting more than 30 closed shells with an average size of 70 nm while the IF-WS2 particles are highly faceted and the diameter of their hollow void in the center is about half the overall diameter of the nanoparticle. The typical size of the IF-WS2 particles is between 70 and 120 nm.

The aim of this paper is to focus on the behavior of IFMoS 2 nanoparticles at the nanoscale. The results obtained with IF-WS2 are reported in (I. Lahouji, et.al.).

ILLUSTRATIVE RESULTS
Figure 2 illustrates a typical behavior of single IF-MoS2 particle of about 60-nm diameter under pressure. The corresponding force-time curve recorded during the compression test is reported in figure 3. During the loading stage the particle first accommodates the contact pressure (see Figure 2(b)-(c)). This stage is correlated with a progressive increase of the normal force (see Figure 3). When the maximum force is reached (8µN at t=11s), corresponding to an estimated pressure contact of 2.2 GPa, the particle starts to exfoliate (zoomed part of Figure 2(d)) and then slides. Figure 2(e) shows that, under the combined effect of the shear stress induced by the sliding of the particle in the contact and the pressure applied on the particle, the fullerene exfoliates. On the force-time curve (see Figure 3), the sudden sliding of the particle results in a sudden drop of the load. At the end of the test, the shape of the particle is preserved and only some external layers are exfoliated (see Figure 2(g)-(h)-(i)).

Figure 4 shows a series of image captures obtained from the video recorded during the sliding test with a single MoS2 particle of about 80-nm diameter. In this experiment, the Si substrate was moved sideways to introduce a shear force, where as a constant normal force of 1 µN corresponding to pressure stress of 300 MPa was applied to the particle. It should be noted that it is not possible to measure the tangential force with this equipment.

The sequence clearly shows that the particle under relatively low pressure of 300 MPa is able to roll when the Si substrate is sliding, leading to a delamination of the outer sheets (zoomed parts of Figures 4(g)-(h)).

SUMMARY
For the first time, real-time imaging of the behavior of individual MoS2 nanoparticles in a dynamic contact clearly shows their deformation during loading process followed by an exfoliation of the outer sheets.

Under compression, the exfoliation of MoS2 layers was observed for a contact pressure estimated at 2.2 GPa.

These preliminary results are encouraging to go deeper into the better understanding of how particles are deformed during mechanical solicitation at the nanoscale. Therefore, further work are in progress to establish a correlation between the behavior of single MoS2 and WS2 nanoparticles during mechanical solicitation at the nanoscale and their lubricating properties observed at macroscale. The objective is to improve our understanding of the lubrication mechanisms of fullerenes.

ACKNOWLEDGMENTS
This work was supported by EU 7th framework programme ADDNANO (NMP4 –LA-2009-229284). Thanks are also due to the CLYM—http://clym.insa-lyon.fr—for the access to the 2010F microscope.